1. Field of the Invention
The invention relates to a method for fabricating semiconductor device, and more particularly, to a method of removing hard mask layer on a gate structure while forming a protective layer on the surface of a substrate.
2. Description of the Prior Art
In order to increase the carrier mobility of semiconductor structure, it has been widely used to apply tensile stress or compressive stress to a gate channel. For instance, if a compressive stress were to be applied, it has been common in the conventional art to use selective epitaxial growth (SEG) technique to form epitaxial structure such as silicon germanium (SiGe) epitaxial layer in a silicon substrate. As the lattice constant of the SiGe epitaxial layer is greater than the lattice constant of the silicon substrate thereby producing stress to the channel region of PMOS transistor, the carrier mobility is increased in the channel region and speed of MOS transistor is improved accordingly. Conversely, silicon carbide (SiC) epitaxial layer could be formed in silicon substrate to produce tensile stress for gate channel of NMOS transistor.
Conventionally, etching process is conducted to remove hard mask from the gate structure during the formation of epitaxial layer, and then contact plugs are formed thereafter. However, etchant used during the etching process often damages the surface of epitaxial layer and affects the performance of the device. Hence, how to improve the current fabrication to resolve this issue has become an important task in this field.